The present invention relates to a magnetic random access memory (MRAM) device, and more particularly, to an MRAM element that can be switched by voltage and a method for using the same.
Magnetic random access memory (MRAM) is a new class of non-volatile memory, which can retain the stored information when powered off. An MRAM device normally comprises an array of memory cells, each of which includes at least a magnetic memory element and a selection transistor coupled in series between appropriate electrodes. Upon application of a switching current or magnetic field to the magnetic memory element, the electrical resistance of the magnetic memory element would change, thereby switching the stored logic in the respective memory cell.
FIG. 1 shows a conventional MRAM element comprising a magnetic reference layer 12 and a magnetic free layer 14 with an insulating tunnel junction layer 16 interposed therebetween. The magnetic reference layer 12, the insulating tunnel junction layer 16, and the magnetic free layer 14 collectively form a magnetic tunneling junction (MTJ) 18. The magnetic reference layer 12 and free layer 14 have magnetization directions 20 and 22, respectively, which are substantially perpendicular to the respective layer planes. Therefore, the MTJ 18 is a perpendicular type comprising the magnetic layers 12 and 14 with perpendicular anisotropy. The magnetization direction 22 of the magnetic free layer 14 can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction 20 of the magnetic reference layer 12. The insulating tunnel junction layer 16 is normally made of an insulating material with a thickness ranging from a few to a few tens of angstroms. However, when the magnetization directions 22 and 20 of the magnetic free layer 14 and reference layer 12 are substantially parallel, electrons polarized by the magnetic reference layer 12 can tunnel through the insulating tunnel junction layer 16, thereby decreasing the electrical resistivity of the perpendicular MTJ 18. Conversely, the electrical resistivity of the perpendicular MTJ 18 is high when the magnetization directions 20 and 22 of the magnetic reference layer 12 and free layer 14 are substantially anti-parallel. Accordingly, the stored logic in the magnetic memory element can be switched by changing the magnetization direction 22 of the magnetic free layer 14.
There are several ways to switch the magnetization direction 22 of the magnetic free layer 14 in MRAM devices. In a field-toggle MRAM device, the magnetization direction 22 of the magnetic free layer 14 is switched by a magnetic field induced by a “write” word line. The field-toggle MRAM requires the extra write word line circuitry and a substantially large current to generate the magnetic field for switching, thereby rendering it impractical for high density memory applications. In a spin transfer torque MRAM device, the magnetization direction 22 of the magnetic free layer 14 is switched by a spin polarized current, thereby eliminating the need for the write word line circuitry. However, the switching current of the spin transfer torque MRAM may still be too high to prevent the miniaturization of the selection transistor and hence the memory cell. Moreover, the spin transfer torque MRAM requires lower memory resistance because of the high switching current passing therethrough, thereby necessitating the insulating tunnel junction layer 16 to be thinner and less reliable.
A recent study by Wang et al. on perpendicular MTJ shows that the perpendicular anisotropy of magnetic layers in magnesium oxide (MgO) based MTJ structures can be changed by the voltage applied to the magnetic layers. See Wei-Gang Wang et al., “Electric-field-assisted switching in magnetic tunnel junctions,” Nature Materials Vol. 11, 64-68 (2012).
In the test set-up of Wang et al. as shown in FIG. 1, a positive electric potential is applied to the MTJ 18 to drive electrons into the magnetic free layer 14 made of 1.6 nm thick Co40Fe40B20. When the insulating tunnel junction layer 16, which is made of 1.4 nm thick magnesium oxide (MgO), is sufficiently thick and the electrical resistance across the MgO junction layer 16 is sufficiently high, the current density through the MgO junction layer 16 will be low. In this case, the magnetic reference layer 12, which is made of 1.3 nm thick Co40Fe40B20, and the magnetic free layer 14 adjacent to the MgO junction layer 16 effectively form a parallel plate capacitor with the MgO junction layer 16 acts as the dielectric. When a voltage is applied to the MTJ 18, electrical charges will accumulate in the two magnetic layers 12 and 14 like a capacitor, resulting in formation of an electric field 24 across the MgO junction layer 16. The applied positive voltage as shown in FIG. 1 causes the magnetic free layer 14 to have a negative potential, i.e. electron accumulation at the interface between the magnetic free layer 14 and the MgO junction layer 16. With increasing applied voltage and electron accumulation at the interface, the magnetic free layer 14 shows decreasing perpendicular anisotropy, which is reflected by decreasing coercivity field Hc. The decreasing of perpendicular anisotropy and corresponding coercivity field would facilitate the switching of the variable magnetization direction 22 of the magnetic free layer 14 from parallel to anti-parallel orientation. In contrast, electrons will be depleted at the interface between the magnetic reference layer 12 and the MgO junction layer 16 with increasing applied voltage, resulting in increasing perpendicular anisotropy and coercivity field for the magnetic reference layer 12. It should be noted that Wang's finding can only be used to help switching the variable magnetic moment 22 of the magnetic free layer 14 from parallel to anti-parallel orientation, not the other way, i.e. anti-parallel to parallel orientation.
A possible explanation for the observed changes in perpendicular anisotropy with electron accumulation/depletion at interfaces between the magnetic free layer/MgO junction layer/magnetic reference layer as reported by Wang et al. may be that having increased amount of electrons at the interface between the magnetic free layer 14 and the MgO junction layer 16 allows more conductive electrons to fill the 3d-band of CoFe lattice and reduce the unpaired 3d-valence electron population, thereby making the broken-symmetry induced surface perpendicular anisotropy weaker and the magnetic free layer 14 magnetically softer. When 3d-electrons are depleted at the interface between the magnetic reference layer 12 and the MgO junction layer 16, electrons will be depleted first from paired 3d-electrons according to Hunt's Rules. As more 3d-electrons become unpaired in the magnetic reference layer 12, the surface perpendicular anisotropy thereof increases, making the magnetic reference layer 12 magnetically harder to switch by external field or spin transfer torque.
While a conventional MTJ having an MgO junction layer can exhibit the above-described electric field assisted switching effect, the effect is not significant because the MgO junction layer is thin enough to allow a relatively high density of electrons to tunnel therethrough, thereby minimizing the capacitive effect needed to generate electrons at the interface between the magnetic layer and the MgO junction layer. Increasing the MgO thickness can improve the capacitive effect and the tunneling magnetoresistance (TMR) but would also adversely increase the MTJ resistance, thereby increasing the power consumption. Moreover, the prior art method illustrated in FIG. 1 does not completely eliminate the need for a switching current and can only help switching a magnetic free layer from parallel to anti-parallel orientation, i.e. low resistance to high resistance state.
For the foregoing reasons, there is a need for an MRAM device having MTJ memory elements that can be easily switched and a method for switching the memory elements between low and high resistance state.